Servers that provide games to portable devices over communication networks have been widespread in recent years. Games provided by such servers include those in which multiple users can participate (so-called “social games”), and among such games there are known those which not only enable users to battle against or cooperate with each other but also allow them to communicate with each other (for example, refer to Japanese Unexamined Patent Publication No. 2005-034303).
As an example of such social games, a game is also known in which a user battles against another user by using, for example, cards (so-called “card battle games”) (for example, refer to Japanese Unexamined Patent Publication No. 2008-142352). In a card battle game, each user can hold an arbitrary number of cards. Further, by using at least one held card, each user forms a set of cards (“deck”) to be used for the battle. Each card has parameters such as offensive power, defensive power, health point, etc., and, based on the parameter values of the cards forming the deck, the user tries to reduce the health point parameter value of the other user. The other user also, based on the parameter values of the cards forming the deck, tries to reduce the health point parameter value of the user. The users thus stage an attack against each other in turn and, when the health point parameter value of one of the users is reduced to zero, the battle ends.
In such a card battle game, it is common practice to determine the total parameter value of the deck by simply adding up the parameter values of the cards forming the deck. It has also been common, for example, to change the parameter values of the cards according to the number of cards forming the deck or the combination of the cards.